1. The Field of the Invention
This application relates generally to fasteners. More specifically, the present invention relates to a medical fastener having a head optimized to transmit torque and to limit the possibility of damage to the head or threads during use.
2. The Relevant Technology
Conventional fasteners, such as screws, are used in a wide range of applications, ranging from wood working to medical procedures. Different screw head geometries have been developed to transmit torque from a driver to the screw head and the screw shaft. For metallic screws the most popular types of head are slotted heads or cruciform heads. A blade-type driver is typically used with a slotted head screw, while a cruciform-type driver is used with a cruciform head.
Although these are the common types of screw head, they suffer from significant problems, especially when used for medical procedures. For instance, during use of a slotted head screw it is typical for the blade-type driver to slide out of the slot resulting in damage to the materials surrounding the screw. This can be detrimental to the surrounding bone or tissue into which the screw is placed. With respect to the cruciform-type screws, the forces applied to the screw by way of the cruciform-type driver can result in the tip of the driver being forced out of engagement with the screw. In addition, the limits of torque transfer associated with the cruciform-type screw can prevent the screw from being fully seated during use. In either case, this can be detrimental to a beneficial outcome to bone growth and rehabilitation of the patient.
Due to the high mechanical properties of metals, metallic screws can include a slot head or cruciform head to enable the transfer of the high torques. Unfortunately, for polymeric screws it is difficult to apply the desired torque to secure the screw, because the head of the screw will typically be stripped under high torques. This is especially true for biodegradable screws for medical applications. It is often impossible to apply adequate axial force to a biodegradable screw when attempting to fixate small bone fragments or thin bone structure. A simple slot is not optimal since the driver blade can slide out of the screw head and damage the surrounding tissue. A recessed head offers more surface area due to it conical shape, but since plastics and in specific biodegradable polymers have a low young modulus the driver blade can over-wind and destroy the screw head.
In addition to the above, problems can occur with current screws that prevent the screw from being completely driven into the bone or structure. To secure a good fixation of the screw in a bone the hole in the bone is usually tapped to create a threaded hole. There is a tendency that blood and scale of bone will be in the threaded hole. If a screw is screwed into the threaded hole the scale of bone, particles and blood will be pressed between the screw and the threads formed in the threaded hole and will increase the frictional contact between the screw and the threads. Depending on the amount of material remaining in the thread hole the generated friction can be so high that it is not possible to mount the screw within the threaded hole without destroying the screw head or the threads.